(a) Antibacterial Drugs and Clostridium difficile 
The development of antibacterial drugs represents one of the most important medical advances of the 20th Century. Previously untreatable diseases could now be readily controlled and it was felt that many diseases would be eradicated with these new wonder drugs. Despite these significant advances in treatment, infectious diseases are the third major cause of mortality in the USA (Clin. Infect. Dis., 2004, 38, 1279-1286) and remain one of the most significant global healthcare problems. Rates of resistance in all of the major pathogenic bacteria are rising dramatically and of particular concern is the increasing number and severity of nosocomial infections (Infectious Disease Society of America, 2004, Bad Bugs, No Drugs). The emergence of multi-drug resistant pathogens has rendered many of the current frontline drugs completely ineffective in controlling many diseases.
A particular subset of bacterial pathogens of concern is those classified as spore-forming bacteria. Bacterial spores (endospores) are dormant, non-reproductive structures formed by bacteria in response to environmental stress. Once environmental conditions become favourable, the spores germinate and the bacteria proliferate. In the case of pathogenic bacteria, germination in a human host may result in disease.
Bacterial spores are extremely tolerant to many agents and environmental conditions including radiation, desiccation, temperature, starvation and chemical agents. This natural tolerance to chemical agents allows spores to persistent for many months in key environments such as hospitals, other healthcare centres and food production facilities, where standard cleaning agents, germicides and sterilisation processes do not eradicate the bacteria. In the case of food production, the presence of spores can have significant consequences ranging from simple food spoilage through to the spread of food-borne pathogens and food poisoning. More recently, attention has been drawn to the risks associated with the spores of Bacillus anthracis, the causative agent of anthrax. The spores can be readily prepared as a dry powder that can be disseminated by numerous methods and used as a bioterrorist agent. Anthrax is considered the single most worrying bioterrorism agent (CDC Emerg. Infect. Dis., 2004, 5 (4), 552-555). This can be highlighted by the postal anthrax attacks in the United States in 2001. There were 22 confirmed infections resulting in 5 deaths with the cost of cleanup and decontamination following the attacks estimated at $1 billion.
Important spore-forming bacteria are the Gram-positive endospore-forming bacteria of the genera Bacillus and Clostridium. Examples of the genus Bacillus of health concern to humans include, but are not limited to, B. anthracis and B. cereus. Bacillus anthracis is of particular concern as the causative agent of anthrax. Anthrax infection can occur through ingestion, inhalation or cutaneous contact with Bacillus anthracis spores resulting in three distinct clinical forms. Cutaneous infection accounts for about 95% of all infections and is generally well controlled with the use of suitable antibiotics. Around 20% of untreated cases of cutaneous anthrax will result in death. Intestinal infection is characterized by an acute inflammation of the intestinal tract resulting in nausea, loss of appetite, vomiting, fever, abdominal pain, vomiting of blood and severe diarrhoea. Intestinal anthrax results in death in 25% to 60% of cases. The most severe form of the disease is pulmonary anthrax which is often fatal, even with aggressive and timely antibiotic administration. The ability to readily disperse anthrax spores through the air and over a wide area to induce pulmonary anthrax makes anthrax the primary bioterrorism agent.
Members of the genus Clostridium are Gram-positive, spore-forming, obligate anaerobes. Exemplary species causing human disease include, but are not limited to, C. perfringens, C. tetani, C. botulinium, C. sordellii and C. difficile. Clostridia are associated with diverse human diseases including tetanus, gas gangrene, botulism and pseudomembraneous colitis and can be a causative agent in food poisoning.
Of particular concern is disease caused by Clostridium difficile. Clostridium difficile causes Clostridium difficile-associated diseases (CDAD) and there has been a ten-fold increase in the number of cases within the last 10 years, with hyper-virulent and drug resistant strains now becoming endemic. Recent HPA figures show there were 55,681 cases of C. difficile infection in patients aged 65 years and above in England in 2006 (up 8% on the previous year). Perhaps most worrying are the cases of CDAD with no underlying antibiotic use now being reported.
Clostridium difficile is a commensal enteric bacterium, the levels of which are kept in check by the normal gut flora. However, the bacterium is the causative agent of C. difficile-associated disease (CDAD) and has been identified as the primary cause of the most serious manifestation of CDAD, pseudomembraneous colitis. CDAD is associated with a wide range of symptoms ranging from mild diarrhoea through to pseudomembraneous colitis, toxic megacolon and death. The primary risk factor for the development of CDAD is the use of antibiotics disrupting the normal enteric bacterial flora causing an overgrowth of Clostridium difficile. Although clindamycin is the major antibiotic associated with CDAD, the disease is now associated with nearly all antibiotics including members of the fluoroquinolone, cephalosporin, macrolide, β-lactam and many others classes.
CDAD is primarily of concern in the hospital setting and is of particular concern amongst elderly patients where mortality rates are particularly high. Reported rates of CDAD have increased dramatically in recent years with over 55,000 cases reported in the UK in 2006 (Health Protection Agency Surveillance of Healthcare Associated Infections Report 2007).
Mortality rates in the USA have risen from 5.7 per million of population in 1999 to 23.7 per million in 2004. Colonisation rates of C. difficile in the general population are up to 3% although hospitalisation dramatically increases the rates of colonisation up to 25%. Of particular concern is the emergence of new endemic strains. A particularly pertinent example is the hyper-virulent BI/NAP1 (also known as ribotype 027) strain which shows increased toxin A and B production as well as the production of additional novel binary toxins.
A critical factor associated with clostridia is the high rates of bacterial spores present in hospital environments. It has recently been shown that many of the standard hospital cleaning agents in use are ineffective at eradicating clostridial spores for the environment resulting in ineffective disease control (Infect Cont. Hosp. Epidemiol., 2007, 28, 920-5). The hyper-sporulation characteristics of strains such as BUNAP1 contribute significantly to the issue.
Although the primary risk factors associated with CDAD are underlying antibiotic use and age (CMAJ, 2008, 179 (8), 767-772; J. Antimicrob. Chem., 2003, 51, 1339-1350) there are numerous other associated factors including for example the use of proton pump inhibitors, use of H2 receptor antagonists, use of diuretics, length of hospital stay, use of feeding tubes, mechanical ventilation and underlying co-morbidity.
Gastric acidity is part of the natural defence mechanism against ingested pathogens and any reduction in the acidity of the stomach can result in colonisation of the normally sterile upper gastrointestinal tract which can result in a disturbance of the normal enteric microflora. As such, the use of gastric acid suppressive agents, such as proton pump inhibitors (PPIs) and histamine H2-receptor antagonists (H2RAs) is associated with an increased risk of C. difficile colonisation and subsequent development of CDAD. The use of PPIs and H2RAs has previously been associated with other enteric infections such as traveller's diarrhoea, salmonellosis and cholera. Dial et al. have reported that the risk of CDAD increases with the use of gastric acid suppressive agents in both the community (JAMA, 2005, 294(23), 2989-2995) and hospital settings (CMAJ, 2004, 171(1), 33-38).
PPIs include, but are not limited to, omeprazole (Losec, Prilosec, Zegerid), lansoprazole (Prevacid, Zoton, Inhibitol), esomeprazole (Nexium), pantoprazole (Protonix, Somac, Pantoloc, Pantozol, Zurcal, Pan) and rabeprazole (Rabecid, Aciphex, Pariet, Rabeloc).
H2RAs include, but are not limited to, cimetidine (Tagamet), ranitidine (Zinetac, Zantac), famotidine, (Pepcidine, Pepcid), roxatidine (Roxit) and nizatidine (Tazac, Axid).
Triple therapy with PPIs or H2RAs together with a combination of two antibiotics is a recognised treatment for the eradication of Helicobacter pylori infections (Aliment. Pharmacol. Ther., 2001, 15(5), 613-624; Helicobacter., 2005, 10(3), 157-171). However, there are a few reports that this triple therapy regimen can lead to CDAD side effects (Am. J. Gastroenterol., 1998, 93(7), 1175-1176; J. Int. Med., 1998, 243(3), 251-253; Aliment. Pharm. Ther., 2001, 15(9), 1445-1452; Med. Sci. Monit., 2001, 7(4), 751-754). Typical antibacterials used to treat Helicobacter pylori infections are a combination of agents selected from, but not limited to metronidazole, amoxicillin, levofloxacin and clarithromycin—many of which are strongly associated with the development of CDAD. Current therapies are extremely limited; particularly in view of the fact nearly all antibiotic classes are associated with causing the disease. The only FDA approved drug for treatment of CDAD is vancomycin although metronidazole is also extensively used. Widespread vancomycin use for the treatment of CDAD is of concern due to its bacteriostatic action against clostridia, relatively high cost and the possible selection of resistant C. difficile strains as well as other bacteria (particularly Enterococcus spp.). A key issue with both metronidazole and vancomycin is the high relapse rate with at least 20% of patients experiencing at least one recurrent episode. Relapse is proposed to occur due to the inability to eradicate the clostridium spores during therapy resulting in subsequent outgrowth to a pathogenic state. This inability to control spore formation allows for continued contamination of the hospital environment. As such, agents able to eradicate vegetative cells and control endospores would be of significant advantage.
The primary therapy option for the treatment of CDAD is discontinuation of any current antimicrobial treatment followed by appropriate use of either vancomycin or metronidazole. Both agents are usually administered orally although metronidazole may also be administered intravenously and in severe cases, vancomycin may also be administered via numerous other routes including intracolonic, through nasal gastric tube or as a vancomycin-retention enema. Additional antibiotics agents that have been reported to be used in the treatment of CDAD include fusidic acid, rifamycin and its analogues, teicoplanin and bacitracin although none show particular efficacy over vancomycin or metronidazole. In addition to halting any offending antibacterial treatment, the use of antiperistaltic agents, opiates, or loperamide should be avoided since they can reduce clearance of the C. difficile toxins and exacerbate toxin-mediated colonic injury. Such agents may also precipitate ileus and cause toxic dilation of the colon (J. Med. Microbiol., 2005, 54, 101-111; JAMA, 1993, 269, 71-5; Postgrad. Med. J., 1990, 66(777), 582).
Alternative therapies, used as standard alone agents or in conjunction with antibacterials, are aimed at either trying to re-establish the native gut microorganism population, reducing the levels of C. difficile toxins or stimulating the immune system (for reviews see Antibiotic Treatment for Clostridium difficile-Associated Diarrhea in Adults, Cochrane Database of Systematic Reviews 2007, Issue 3. Art. No.: CD004610.; Clin. Inf. Dis., 2008, 46(S1), S32-S42; Clin. Inf. Dis., 2007, 45(S2), S122-S128; J. Med. Microbiol., 2005, 54, 101-111 and references therein). Thus, alternative CDAD therapies include provision of Saccharomyces boulardii or Lactobacillus acidophilus in conjunction with antibiotics, faecal transplantation and in severe cases where all other therapy options have failed, surgery. Although rates of colectomy are low (up to 3% of cases) it is associated with high mortality rates (up to 60%).
As such, there is a pressing need for new and effective agents to treat diseases associated with spore forming bacteria, particularly those caused by members of the genera Clostridium and Bacillus and in particular disease associated with Clostridium difficile infection. This need is particularly acute in the light of the refractory nature of Clostridium difficile to many broad spectrum antibiotics (including β-lactam and quinolone antibiotics) and the frequency with which resistance emerges (Antimicrob. Agents Chemother., 1985, 28(6): 842-844).
(b) Prior Art
WO2007056330, WO2003105846 and WO2002060879 disclose various 2-amino benzimidazoles as antibacterial agents.
WO2007148093 discloses various 2-amino benzothiazoles as antibacterial agents.
WO2006076009, WO2004041209 and Bowser et al. (Bioorg. Med. Chem. Lett., 2007, 17, 5652-5655) disclose various substituted benzimidazole compounds useful as anti-infectives that decrease resistance, virulence, or growth of microbes. The compounds are said not to exhibit intrinsic antimicrobial activity in vitro.
U.S. Pat. No. 5,824,698 discloses various dibenzimidazoles as broad-spectrum antibiotics, disclosing activity against both Gram-negative and Gram-positive bacteria, including Staphylococcus spp. and Enterococcus spp. However, this document does not disclose activity against anaerobic spore-forming bacteria and in particular does not disclose activity against any Clostridium spp. (including C. difficile).
US 2007/0112048 A1 discloses various bi- and triarylimidazolidines and bi- and triarylamidines as broad-spectrum antibiotics, disclosing activity against both Gram-negative and Gram-positive bacteria, including Staphylococcus spp., Enterococcus spp. and Clostridium spp. However, this document does not disclose compounds of general formula (I) as described herein.
Chaudhuri et al. (J. Org. Chem., 2007, 72, 1912-1923) describe various bis-2-(pyridyl)-1H-benzimidazoles (including compounds of formula I as described herein) as DNA binding agents. This document is silent as to potential antibacterial activity.